Conventional overload relays for protecting electric motors against excessive current are of the thermal type wherein a bimetal actuator is heated directly or indirectly as a function of motor current and operates to effect interruption of the motor circuit under overload conditions. In general, industrial motors are adequately protected by such bimetal relays. Disadvantageously, however, for applications in which high duty cycles are required or the load may vary widely, bimetal relays do not accurately represent the thermal characteristics of a motor, especially under transient conditions. Such a mismatch between the thermal characteristics of a motor and a relay may cause nuisance trips if the overload threshold is set too conservatively or, conversely, motor overheating if the threshold is set to avoid nuisance trips. Moreover, improvements in the motor industry which have resulted in a reduced weight-to-horsepower ratio have consequently resulted in higher motor operating temperatures, thereby reducing the overload capability of motors. It would be desirable to provide an overload relay which is not susceptible to the aforementioned deficiencies of conventional overload relays.
Accordingly, it is an object of the present invention to provide a relay system which is capable of accurately duplicating thermal characteristics of a motor, thereby adequately protecting the motor, even under transient conditions.
Another object of the present invention is to provide a relay system which is capable of maintaining the thermal history of a motor.
Another object of the present invention is to provide an overload relay system which is capable of protecting a motor from: phase faults, including phase sequence errors, phase unbalance, and phase losses; ground faults; load losses; and load jams.
Still another object of the present invention is to provide a microprocessor-based relay system including a data base for storing heating and cooling curves for a wide variety of motors.
Yet another object of the present invention is to provide a microprocessor-based relay system which operates in a start-up mode that allows for high inrush currents during starting transients without tripping the relay.